This invention relates to the removal of carbonyl sulfide from an organic liquid by selective adsorption of the carbonyl sulfide on an alumina adsorbent and the regeneration of the alumina adsorbent. 2. Description of the Related Art
Carbonyl sulfide (COS) is an undesirable impurity in materials such as, for example, petroleum hydrocarbons because the COS is a sulfur source and therefore a potential atmospheric pollutant. COS also acts as an undesirable contaminant of industrial processes such as, for example, by poisoning of polymerization catalysts when the COS is present in petroleum-derived polymerizable olefins such as propylene. COS may be introduced into such processes as a contaminant initially present in the feedstock or it may be formed in the treating process as a result of the molecular sieve-catalyzed reaction of carbon dioxide with hydrogen sulfide or other sulfur compounds.
Depending upon the process and the required purity of the product, the COS level in the starting material may be required to be reduced to below 1 part per million by weight (ppmw) and sometimes to levels below 100 part per billion by weight (ppbw). Concentration of COS in the range of a few ppmw cannot be separated efficiently from a petroleum feedstock such as propylene by fractional distillation because the boiling point of COS differs from propylene by only 3.4.degree. C.
Khelghatian U.S. Pat. No. 3,315,003 teaches a process for removing COS from a hydrocarbon by first contacting the hydrocarbon with a liquid such as monoethanolamine which scrubs the hydrocarbon to remove acid gases such as H.sub.2 S and CO.sub.2 and part of the COS. The hydrocarbon is then distilled. After several subsequent distillations, the liquid bottom product is treated with a soda-lime to remove any remaining COS.
However, separation of COS by processes which involve distillation, in addition, are extremely costly due to the cost of energy to vaporize virtually all of the liquid. It is, therefore, desirable to provide other means for the removal of COS impurities from organic liquids.
It has also been proposed to remove COS from hydrocarbons by catalytic hydrolysis to form H.sub.2 S, for example, using alumina as a catalyst. Frevel et al U.S. Pat. No. 3,265,757 teaches the hydrolysis of COS contained in a liquid hydrocarbon by contacting a mixture of the liquid hydrocarbon and water, at a temperature of from 20 to 50.degree. C., with a high surface area alkaline, active alumina containing from 0.15 to 3 wt. % of sodium or potassium. The patentees state that the hydrolysis reaction will not commence, however, if the alumina is bone dry. They suggest either moistening the alumina catalyst with ion-free water prior to the reaction or passing a mixture of ion-free water and the liquid hydrocarbon through the catalyst bed until a sufficient amount of water has built up on the alumina to permit the hydrolysis reaction to proceed. However, while this process does remove COS (by converting it to H.sub.2 S), it does not remove sulfur per se from the hydrocarbon, but merely changes the form of the sulfur compound which still must be subsequently removed from the hydrocarbon by another process step.
In a later patent dealing with the same type of reaction, Polleck et al U.S. Pat. No. 4,491,516 teach that the reaction rate for the hydrolysis of COS with water over alumina may be greatly increased if the ratio of water to COS ranges from 1 to 10 moles of water per mole of COS, preferably 1.5 to 6 moles of water per mole of COS, or about 30% of saturation of the hydrocarbon, whichever upper limit provides the lesser amount of water.
Brownell et al U.S. Pat. No. 4,455,446 teaches the removal of COS from propylene by hydrolysis over a catalyst comprising platinum sulfide on alumina. The patentees state that the hydrolysis reaction may be carried out in either the gaseous or liquid phase with a temperature of 35 to 65.degree. C. used for the liquid phase. An amount of water at least double the stoichiometric amount of the COS to be hydrolyzed must also be present.
Harris et al U.S. Pat. No. 4,391,677 describes a process for desulfurizing a butene-1 rich stock containing sulfurous impurities such as H.sub.2 S, COS, and CH.sub.3 SH. The process comprises passing the feed stream through a desulfurization zone maintained under desulfurization conditions and containing a charge of at least one desulfurization medium capable of adsorbing, absorbing, or converting H.sub.2 S, COS, and CH.sub.3 SH to high boiling sulfurous compounds. The thus-treated feed stream, now essentially free from H.sub.2 S, COS, and CH.sub.3 SH, is then passed to a distillation zone, and recovered as a bottom product as a butene-2 rich stream containing high boiling sulfurous compounds. The desulfurization zone comprises a bed of activated alumina followed by a bed of zinc oxide. The activated alumina is said to hydrolyze COS in the presence of 20 to 1000 ppm of water to H.sub.2 S and partially to remove H.sub.2 S and methyl mercaptans. The zinc oxide is said to remove all the H.sub.2 S and methyl mercaptan not removed by the alumina bed.
COS has also been removed from liquid hydrocarbons by adsorption on a zeolite adsorbent. Collins U.S. Pat. No. 3,654,144 discloses removing COS by adsorbing it on a particular modified zeolite A adsorbent comprising an alkali metal cation form of zeolite A which has been ion-exchanged with alkaline earth metal cations, preferably calcium cations, to the extent of from 20 to about 100 equivalent percent.
Innes U.S. Pat. No. 4,098,684 describes the removal of COS and other sulfur compounds by passing them through a dual bed of zeolites comprising, respectively, a 13X molecular sieve, and a zeolite A sieve having a pore size of 4 Angstroms. The commercially available 13X zeolite is said to remove any H.sub.2 S and mercaptans present. The capacity for COS adsorption by the 13X sieve is said to be small. The 13X zeolite is described as a three dimensional network with mutually connected intracrystalline voids accessible through pore openings which will admit molecules with critical dimensions up to 10 Angstroms and having the general chemical formula: 0.83.+-.0.05 Na.sub.2 O/1.00 Al.sub.2 O.sub.3 /2.48.+-.0.038 SiO.sub.2. The molecular sieve beds may be regenerated by passing a hot, substantially nonadsorbable, purge gas through the beds at a temperature of about 177.degree. to 316.degree. C. (350.degree. to 600.degree. F.).
While zeolite materials have thus been used as adsorbing agents to remove sulfurous compounds such as COS from liquid hydrocarbons, it has been found that zeolite, with its cage structure, has a low adsorption rate at ambient temperature and is, therefore, not practical for treating liquid at such temperatures.
It would, therefore, be highly desirable to provide a process for the removal of sulfurous impurities such as COS from liquid hydrocarbons, preferably in the absence of water, using an alumina adsorbent having high adsorption characteristics yet capable of being regenerated without substantial loss of adsorption capability.